1. Field of the Invention
The present invention relates generally to the construction and use of catheters for interventional and diagnostic procedures. In particular, the present invention relates to methods and apparatus for advancing catheters through restrictions and occlusions within body lumens and cavities.
Atherosclerosis is a form of arteriosclerosis characterized by irregularly distributed deposits on the walls of a patient's arteries. Such deposits frequently fibrose and calcify over time, seriously compromising the patient's health.
A number of catheter-based approaches have been developed for diagnosing and treating atherosclerosis and other forms of arteriosclerosis. The most common interventional technique for treating atherosclerosis is balloon angioplasty, in which a balloon-tipped catheter is introduced into the vascular system and the balloon expanded within a region of stenosis. Other interventional techniques include atherectomy, where, for example, a catheter having a cup-shaped rotating cutter is introduced into the vascular system and used to sever and capture at least a portion of the stenotic material. Other interventional techniques include laser ablation, mechanical abrasion, chemical dissolution, and the like. Catheter-based diagnostic techniques include ultrasonic imaging, in which an ultrasonic transducer disposed at the distal end of a vascular catheter is introduced to the region of stenosis.
With most of these techniques, it is necessary to advance the distal end of the catheter at least partly through the stenosed region before the interventional or diagnostic procedure can be commenced. While such initial advancement is often not a problem, it can be very problematic when the occlusion is severe and little or no lumen remains to receive the catheter. Under such circumstances, it is necessary to at least partly recanalize (create an opening through) the occlusion before the catheter procedure can begin.
A number of methods for recanalizing severe occlusions have been proposed, including the use of hot-tipped catheters, laser catheters, and drill-tipped catheters. In general, these approaches rely on very aggressive treatment of the stenotic material to open up a passage. Such aggressive techniques can expose the blood vessel wall to significant injury, for example, vessel perforation. The risk of injury is exacerbated by the unconstrained path that the catheter can follow.
An improved technique for advancing an angioplasty catheter into and optionally through severe occlusions is described in U.S. Pat. No. 4,998,933 (the entire disclosure of which is hereby incorporated herein by reference), which has common inventorship with the present application. A first electrode is disposed at or near the distal tip of the angioplasty catheter and a second electrode is provided on an electrically conductive guidewire. After the guidewire is at least partly advanced into the stenotic material, a high frequency voltage can be applied between the guidewire electrode and the catheter tip electrode in order to generate heat within the stenotic material lying between said electrodes. The stenotic material is softened as it is heated, thereby allowing easier advancement of the angioplasty catheter.
Although a substantial improvement in the art, the catheter described in U.S. Pat. No. 4,998,933 can cause unwanted short circuiting of electrical energy through the blood and blood vessel wall during the application of the high frequency voltage. The catheter employs a single discrete electrode at its distal tip. So long as the tip electrode fully contacts the stenotic material, the induced heat will be substantially limited to the stenotic material. If the electrode is exposed to the blood vessel wall or blood, however, current will flow through the blood vessel tissue or blood, causing the undesired electrical shorting. Moreover, since both the blood vessel wall and the blood have higher electrical conductivities than the stenotic material, they will carry the current in preference to the stenotic material.
Additionally, the prior art catheters provide no means for steering the tip of the catheter away from the wall of the lumen. By way of example, the stenotic material may be deposited asymmetrically within the vessel lumen resulting in an eccentric stenosis. Conventional catheters will proceed along the residual opening in the lumen, thereby risking damage to the exposed healthy vessel wall if balloon dilatation, thermal, or mechanical means are used to increase the lumen diameter. Also, the occluded vessel may not be straight (e.g., in coronary arterial vessels) necessitating a change in the direction of the advancing catheter tip to avoid thermal or mechanical damage to the vessel wall.
For these reasons, it would be desirable to provide improved apparatus and methods for advancing vascular catheters into and past occlusions in blood vessels and other body lumens. In particular, it would be desirable to provide improvements to catheters generally of the type described in U.S. Pat. No. 4,998,933, to enable the catheter to heat the atheromatous material more selectively. It would be further desirable if such catheters were able to discriminate between the atheromatous mass and the blood vessel wall (preferentially heating and ablating the former) so that the catheter would selectively pass through the atheroma as the catheter is advanced through the lumen of the blood vessel. It would be still further desirable if such catheters incorporated means for steering the catheter tip away from the vessel wall (e.g., away from the media and underlying adventitia layer). In order to improve the capability to advance the catheter through partial or total occlusions while minimizing potential damage to the vessel wall, it would be further desirable to provide a catheter with variable stiffness along its length with the greater stiffness in the portions proximal to the balloon and tip and lesser stiffness in the tip region of the catheter to minimize the danger of perforating the vessel wall.
Some of these goals are addressed in co-pending U.S. patent application Ser. No. 07/958,977, filed Oct. 9, 1992 the complete disclosure of which is hereby incorporated herein by reference. The present disclosure provides still further improvement over previous apparatus and methods.
2. Description of the Background Art
U.S. Pat. No. 4,998,933, has been described above. U.S. Pat. No. 2,050,904 describes a hemostatic probe having a tapered cautery tip at its distal end. European Patent Publication 182,689 and U.S. Pat. No. 4,754,752 describe angioplasty balloon catheters having means for internally heating the balloons. A "hot tip" catheter having a metal tip heated by a laser is described in Cumberland et al. (1986) Lancet i: 1457-1459. U.S. Pat. No. 4,654,024, describes a catheter having an electrically heated tip for melting atheroma. U.S. Pat. No. 4,796,622, describes a catheter heaving a tip that is heated by an exothermic reaction. A catheter having a high speed rotating abrasive element at its distal tip is described in U.S. Pat. No. 4,857,046. U.S. Pat. No. 4,709,698 describes the placement of electrode pairs on the surface of a dilatation balloon to heat atheroma as the balloon is expanded. U.S. Pat. No. 4,955,377, describes an RF heating device employing two spaced-apart electrodes disposed within an inflatable angioplasty balloon. U.S. Pat. No. 5,057,105, describes a hot tip catheter.
A means for manually steering a catheter is described in U.S. Pat. No. 4,543,090. That means utilizes a plurality of temperature-activated memory elements in the distal end of the tubular member. The catheter is steered using a manual "joystick" or other similar control means. Yet another means for steering catheters and scopes is described in U.S. Pat. No. 4,934,340 wherein a contractile wire member is shortened by the passage of electrical current through said wire, thereby causing the catheter to deflect.